1) Field of the invention
The present invention relates to apparatus for washing semiconductor wafers with liquid including cleaning chemicals and with pure water.
2) Description of the Prior Art
Washing apparatus for washing semiconductor wafers is known having a plurality of washing tanks positioned in a case, some of which contain therein liquid including cleaning chemicals and other tanks contain therein pure water. The so called chemical tanks and pure water tanks are mutually disposed. In the respective tanks, liquid or pure water is introduced thereinto from the bottom thereof and overflown from the tank, and is circulated by a pump through a filter.
The case in which the washing tanks are positioned have an air duct means defining blowing ports for discharging clean air flowing over the washing tanks and suction ports opposite the blowing ports. The respective washing tanks are disposed between the blowing port and the suction port. The suction port serves to remove the air after passing over the washing tank. Also, washing tanks are integrally formed with an overflow tank which receives pure water or liquid overflowing the washing tank.
These washing tanks within a case are set in a clean room and subjected to a downwardly directed discharge of clean air from the ceiling of the room. This arrangement acts to prevent vapour from cleaning chemical tanks from entering the pure water tanks.
In the respective washing tanks, semiconductor wafers housed in a carrier are dipped in pure water or liquid. Pure water overflows the periphery of the pure water tank after the tank is full of pure water, and then the water within the tank is quickly discharged through a port formed at the bottom of the tank by opening a valve thereof. After that, closing the port with the valve, pure water is again introduced into the tank to overflow and then quickly discharged similarly. After such process is repeated several times, semiconductor wafers within a carrier are transferred into the next cleaning chemical tank to be washed therein, and after that, transferred into the next other pure water tank to be washed in the manner described, and finally into the final washing tank to be washed with pure or super-pure water.
Clean air is apt to receive static electricity because it is of a low degree of humidity, and electrostatic charge will occur on surfaces of semiconductor wafers in accordance with frictions of the wafer surfaces with clean air and pure water. Therefore, the wafer surfaces are apt to be sticked with particles and dust. This results in a lowering of productivity, because the surfaces of semiconductor wafers are sticked with foreign matters or particles contained slightly in clean air and dust generated in the operation of transfer robot or operator.
Otherwise, there has been seen Japanese Patent Laid-open Gazette No.42134/1989 which discloses means for introducing ionized air or gas into washing tanks through pipes and nozzles to thereby neutralize electrostatic charge on the wafer surfaces. However, in the known apparatus, since ionized air is transferred into washing tanks through pipes, the ionization in the air will reduce.